The present invention relates to a method for the production of a single phase silicon carbide material useful in making refractory articles or bodies, such as crucibles, firebricks, furnace linings, and the like. The term "single phase" as used herein means that the silicon carbide material does not contain a substantial amount of additional materials, or phases, which remain in the end product. Thus, the basic starting material is substantially entirely silicon carbide and the end product is substantially entirely sintered silicon carbide.
The chemical and physical properties of silicon carbide make it an excellent material for refractory bodies. These properties include excellent oxidation resistance, very high thermoconductivity, low expansion coefficient, high thermal shock resistance and high strength at elevated temperatures.
There are a number of reasons that refractory materials, for example, firebricks, are not produced of a single phase silicon carbide. Among them are: (1) the cost of manufacture; (2) silicon carbide, especially the alpha form, does not easily self-bond without sintering aids or the use of extremely high temperatures, that is temperatures close to sublimation and which involve recrystallization; (3) silicon carbide refractories typically have excessive porosity, or permeable portions, which can be entered by slags or gases resulting in oxidation damage.
Because of such factors, basic refractory articles typically include additional phases, such as, oxide materials and clays, to alleviate, or minimize, such characteristics. Typical basic refractory articles are fabricated of mixtures of coarse silicon carbide grit bonded with carbon or silica with oxide bonding phase such as clays, silicon oxide or silicon oxynitride, or with a nitride phase such as silicon nitride or aluminum nitride. Such articles are produced commercially by reaction sintering or by hot pressing processes. The main disadvantage of such bonded refractory articles is that the bonding material is present in sufficient amounts that a separate phase is produced. Such multi-phase materials are limited by the physical and chemical properties of whichever phase is most susceptible to the temperature or the environment in which they might be used. Further, oxide bonding phases, by themselves, have low corrosive resistance. Bonding materials such a silica and silicon nitride typically produce articles which have excess porosity, offering areas of attack by reactive gases or liquids.
Silicon carbide is produced in the form of grain or fine powder from which larger bodies or articles are formed by subsequent processing. Until recently silicon carbide particles, particularly the alpha phase, were difficult to densify except by the use of onerous hot pressing techniques. U.S. Pat. Nos. 4,041,117; 4,124,667, and 4,238,434 are illustrative of the more recently discovered methods of producing hard, dense silicon carbide products by so-called "pressureless sintering" processes. In such processes a submicron, or ultrafine, silicon carbide powder is mixed with a sintering aid, such as boron or aluminum, and a slight excess of carbon. The mixture is compacted or shaped and subsequently sintered, without pressure, to produce a high density, sintered product. The products of pressureless sintering have found use in engineering applications, for example, as components for gas turbines, as engine parts and as specialty chemical processing equipment. However, heretofore such components have been fabricated entirely from 100% sinterable silicon carbide powder. The silicon carbide starting material utilized in pressureless sintering processes was substantially entirely of a ultrafine size, preferably submicron, and typically ranging from an average particle size between about 0.1 up to a maximum size of about 5 microns. The use of such costly starting materials is directly reflected in the high cost of such fabricated components.
Refractory articles have also previously been fabricated of recrystallized silicon carbide. A process of recrystallizing silicon carbide is described in U.S. Pat. No. 2,964,823. Recrystallization processes are carried out at comparatively high temperatures, typically in the neighborhood of 2500.degree. C., and require a significant expenditure of energy. The recrystallized products have substantially no shrinkage and are characterized by fairly large pore size and a coarse grain system.